Accurate process control of a paper-making machine requires on-line measurement of certain paper web properties, the most significant of which are basis weight and moisture. These measurements are traditionally achieved by mounting sensors on a platform which scans the paper web transversely of its length. The web can be moving at any speed from a few hundred to several thousand feet per minute.
Microwave moisture sensors of the prior art operate on the principle that water molecules exhibit a resonant absorption, owing to rotational transition at 22 gigahertz (GHz). By monitoring the absorption of energy at this frequency by the water in the web, one obtains a direct measurement of the amount of water present in the web. The principal advantages of this technique are that there is very little interaction between the energy directed to the web and the constituents of the web other than water, and that the sensitivity of the device can be adjusted to span the entire range of water weights found on paper and board-producing machines. Typical sensors of this type are described in U.S. Pat. No. 3,815,019 issued to S. T. Wiles and U.S. Pat. No. 3,851,244 issued to G. R. Mounce.
Measurements obtained in the manner described above are usually presented either as an average of the measurement for one complete traversal across the web or as a graph of the measured value of the variable in question as a function of scanner position across the web. This latter form is known as the measured variable's "profile" and is typically displayed on a video screen or as a visible trace on paper. In some instances, the measured profile is used to make automatic adjustments of process condition across the paper machine, but more often manual control is exercised.
This technique of mechanical scanning has two serious drawbacks. Since the web is moving very rapidly and the scanner relatively slowly, the profile obtained in a typical one-minute scan does not represent a true cross-machine picture, but rather a combination of the true profile and variations along the direction of travel of the web. That is, the scanner actually sees a long diagonal on the web. If the machine is running at 2000 feet per minute, in a one-minute scan, 2000 feet of paper will pass the scanner. To obtain a true profile, the scan must be made extremely rapidly. The scanner, which must operate continuously in a very hostile environment, is a complex and expensive device and tends to be a very high-maintenance item. It is not unusual to find that 40 to 50 percent of the system problems are associated with the scanner. Finally, in many installations there is not sufficient space to mount a scanner and therefore a control system cannot be added to the machine.
In an alternative system, disclosed in U.S. Pat. No. 3,534,260 issued to C. W. E. Walker, a plurality of pairs of microwave transmitters and receivers are successively actuated by ferrite or diode switches to scan a plurality of areas of the web electronically in multiplex fashion. While this technique permits profile measurements of moisture along or across the web without mechanical scanners, the switching components entail additional expense and introduce the problems of poor reliability associated with active devices.